The present invention relates to the field of digital data encryption. More specifically, in one embodiment, the invention provides for the encryption of voice and data transmitted over a digital cellular telephone network to varying levels of security.
Providing encryption for cellular telephone conversations and cellular telephone signalling data is known. For example, U.S. Pat. No. 5,159,634 shows a system for encrypting command messages in a cellular telephone network. In that system, messages are encrypted at a telephone using a secret key, then transmitted to a base station and decrypted there using the secret key. The messages are encrypted in three stages. In the first stage, the message is transformed using the secret key through an invertibis encryption function. In the second stage, the output of the first stage is transformed through an involutory transformation. In the third stage, the output of the second stage is transformed through an encryption function which is the "inverse" of the invertibis encryption function.
An involutory transformation is such that when an input is transformed through the transformation into an output and the output is transformed through the same transformation, the original input results. In the system disclosed in U.S. Pat. No. 5,159,634, the involutory transformation does not depend on any particular key; the particular output depends only on the input and the particular interconnections of the transformation.
One disadvantage of such a system is that a message transformed by the system might be relatively easy to decrypt without even without knowing the secret key ahead of time.
The use of multiple stage encryption systems with more secure decoding is known. For example, the Enigma machines widely used in Germany in World War II used an array of rotors with particular transformations determined by the electrical connections of each rotor. One rotor was a "reflecting" rotor which had the effect of an involutory transformation. The secret keys of the encryption were essentially the choice of rotors, their positions and the internal wiring of the rotors.
In operation, coded messages were generated on one Enigma machine, usually recorded on paper to be transported or transmitted to a message receiver, and decoded by the message receiver using another Enigma machine. The compatibility of the message and the machines used to encode and decode messages was easily solved simply by having all the Enigma machines operate the same way. However, today such simplicity is not possible.
Cellular telephones must be able to operate in many different environments and be compatible with many different manufacturers' equipment, as well as many different regulatory schemes in different countries where cellular telephones are used. For example, in the U.S. the ability for law enforcement agencies to be able to decode messages without secret keys is often given priority over privacy, and therefore the encryption must be reasonably easy to decrypt. However, in other countries, more secure systems might be demanded or required.
Therefore, there is a need for cellular telephone equipment which can be used in a variety of regulatory environments with varying levels of security for message transmission as well as compatibility with cellular telephone standards which may exist.